The invention relates to multimedia signal processing, and in particular relates to hiding auxiliary information in media signals like audio, video and image signals, and using the auxiliary information to detect and characterize tampering of the signal.
Digital watermarking is a process for modifying physical or electronic media to embed a machine-readable code into the media. The media may be modified such that the embedded code is imperceptible or nearly imperceptible to the user, yet may be detected through an automated detection process. Most commonly, digital watermarking is applied to media signals such as images, audio signals, and video signals. However, it may also be applied to other types of media objects, including documents (e.g., through line, word or character shifting), software, multi-dimensional graphics models, and surface textures of objects.
Digital watermarking systems typically have two primary components: an encoder that embeds the watermark in a host media signal, and a decoder that detects and reads the embedded watermark from a signal suspected of containing a watermark (a suspect signal). The encoder embeds a watermark by altering the host media signal. The reading component analyzes a suspect signal to detect whether a watermark is present. In applications where the watermark encodes information, the reader extracts this information from the detected watermark.
Several particular watermarking techniques have been developed. The reader is presumed to be familiar with the literature in this field. Particular techniques for embedding and detecting imperceptible watermarks in media signals are detailed in the assignee""s co-pending application Ser. No. 09/503,881 and U.S. Pat. No. 5,862,260, which are hereby incorporated by reference.
The invention provides methods and systems for detecting and characterizing alterations of media content, such as images, video and audio signals, using watermark messages to carry signal metrics. By comparing signal metrics extracted from the watermark to signal metrics calculated from a suspect signal, a watermark decoder can detect and even classify types of alterations to the watermarked signal. The watermark decoder localizes the alteration to a given block or set of blocks in the suspect image. To illustrate the alteration, the decoder may display a graphical depiction of the suspect signal, along with the block or blocks that are altered as well as the type of alteration associated with those blocks.
Further features will become apparent with reference to the following detailed description and accompanying drawings.